7. The strawberry.—Cut a strawberry down the middle and notice at the base the persistent calyx, in the inside the fleshy receptacle, and on the outside the yellowish nutlets. Open a nutlet with a needle and pick out the seed.
The help of animals.—It has been seen in [Chapter VI.] that insects play a very important part in the fertilisation of many flowers. Very many plants also call in the aid of animals at a later stage for the dispersal of the seeds, and the devices by which this aid is obtained are often very ingenious.
Hooked fruits.—Sometimes after a country walk the reader has probably found small fruits and seeds sticking to his clothes. They have become attached to the cloth by means of small hooks which they carry. The fruit of the herb bennet (wood avens) is a good example of this device. When the stigma of the fruit breaks away, a little hook is left at the top of the style ([Fig. 138]). Goosegrass or cleavers—a common hedgerow plant—and many others have also hooked fruits. Sheep or cattle, grazing near such plants are very likely to brush against them and carry off the fruits and seeds in their hair. They may not again reach the ground until they have been carried far from the place where they grew.
The position of hooked fruits.—It is evident that these little hooks would be quite useless if the fruits grew out of the reach of animals. Such hooked fruits are never found, for instance, on high trees.
Fig. 138.—A, Aggregate Fruit of Wood Avens (nat. size); most of the stigmas have broken off. a, a, a, fruits with stigmas still attached; c, calyx; B, single fruit with stigma still attached (magnified); C, single hooked fruit after loss of stigma (magnified).
Nuts.—A fruit which has a dry, woody pericarp and does not open of itself is called a nut. The fruits of the hazel, oak, and beech are good examples. The shell (pericarp) of the hazel nut ([Fig. 139]) is composed of three layers, and encloses a single seed, or kernel. The nut of the oak ([Fig. 113]) is called an acorn. Its lower part lies in a cup developed from a wrinkled disc by which the lower part of the female flower was surrounded. When the fruit is ripe the nut easily separates from the cup. The acorn contains one seed only, which consists largely of two swollen cotyledons. If the ovary of the female flower of the oak is cut across in June six ovules are found in it. As in the case of the hazel only one ovule is allowed to reach maturity; the rest are sacrificed in order that the remaining one may be more perfectly developed. In the beech, two three-sided nuts occur together, surrounded by a bristly, woody cup, which splits into four valves when the nuts are ripe.
